The invention concerns a method and a device for tightening threaded joints, considering in particular the ergonomic aspects or reaction force characteristics when using manually operated power nutrunners.
Working with a power nutrunner, for instance of the angled type, tend to be rather tiresome for the operator due mainly to the fact that he or she is exposed to the reaction force imparted in the tool housing and handle during each tightening cycle.
In previous studies it has been established that the discomfort to the operator, i.e. the effort needed to counteract the reaction force, depends on the operator's actual working posture, as well as on the character of the reaction force to be handled. Important factors are: magnitude, duration and speed of force build-up and force decrease.
In these studies it has also been shown that the nutrunner together with the arms of the operator respond to the reaction force impulse like a damped mass-spring system with a certain resonance frequency. Accordingly, the duration of the reaction force impulse in relation to the resonance frequency is determining for the resultant amplitude in the nutrunner handle and the discomfort to the operator.
The resonance frequency or system period of the nutrunner-operator arm system depends both on the operator's physical properties and his actual working posture. In other words, the system period is dependent on the ability of the operator to withstand the reaction force impulse.
For a particular person and a certain working posture, the resulting amplitude, which is a measure of the operator's discomfort, depends on the duration of the reaction force impulse. To show by means of an experiment, a half-sine excitation force of different duration has been exerted on a typical nutrunner-operator reaction absorbing system.
The method and the device according to the invention make it possible to:
1. adapt the reaction force impulse to what suits an average operator best for a particular tightening operation, taking into consideration torque applied, joint rate (stiffness), working posture (upright, bending over, upside down etc.)
2. adaptively maintain the reaction force impulse characteristics as the joint stiffness fluctuates.
3. allow easy readjustment to modify the reaction force impulse characteristics to suit different operators (strong/weak) performing the same operations on an assembly line.
As illustrated in FIGS. 1a, 1b, 1c and 2, a short reaction force impulse having a duration equal to 1/4 of the system period (Force F1 FIG. 1a) causes a small amplitude movement only of the operator's arm. See FIG. 2. This is due to the fact that the reaction energy is mainly absorbed by the inertia of the nutrunner itself and to some extent by the inertia of the operator's arm. Such a short lasting reaction force impulse is generated at tightening of a stiff joint, i.e. a joint having a steep torque/rotation characteristic.
FIGS. 1a, 1b, 1c and 2 also illustrate that a long lasting reaction force impulse having a duration equal to a full system period (Force F3 FIG. 1c) causes just a moderate reaction amplitude. At this long lasting impulse, which has slower build-up and ceasing phases, the inertia of the system has no influence upon the reaction impulse absorption. Instead, the operator is able to react in time to be ready to apply muscular force on the nutrunner to counteract the reaction force. This is the case when tightening a so called soft joint, i.e. a joint having a low torque growth/rotation characteristic.
When, however, the joint to be tightened has a medium-soft characteristic (Force F2 FIG. 1b) the inertia of the system has a small influence only on the impulse obsorption, and neither is the operator able to fast enough apply muscle force on the nutrunner handle. Instead, the operator's reaction is late and out of phase with the reaction force impulse, which causes an amplified distortion of the operator's arm and, if repeated, a tiresome discomfort to the operator. See FIG. 2.
Operators tend to describe their tools behavior as "no kick", "kicks", "smooth" or "long pull", which will be explained more in detail. It means roughly that in the "no kick" case, no reaction impulse is felt because the torque impulse is very short. In the "kick" case, the reaction impulse has a typical duration of 0.1-0.3 sec. and is too quick and abrupt for the operator to react to and counteract. Therefore, the "kick" impulse strains the operator physically. "Smooth" means an impulse time of 1.5-2 sec., while longer impulse durations, characterized as "long pull" by the operators, more equals the effort of using a manual wrench.
Impulse duration times which give the most comfortable operation have been found to range between 0.3 and 1.0 sec. dependent on factors like torque level, operator's reaction time, actual working posture, etc.
The examples of reaction impulse times given above are relevant for the most common screw joint sizes: M8 to M10. The described principles apply for both smaller and larger joints, but preferred impulse times vary.
Another discomforting factor for the operator is the occurring variations in the reaction pulse charcteristics of the joints being tightened. It is difficult and tiresome to try to keep up a readiness for reaction impulses of different character.